1. Field of the Invention
This invention relates generally to broadcasting of programs that are coded to help correct received signal errors, and particularly to a scheme that allows a receiver to reproduce a selected program when first tuning to the program with little if any delay caused by such coding.
2. Discussion of the Known Art
Wireless broadcast program signals are subject to fading, or loss of portions of the signals, during transmission of the signals to a broadcast receiver. For example, for radio program signals broadcast in the frequency modulation (FM) band of 88 MHz to 18 MHz, and with no direct line-of-sight path to the receiver, so-called Rayleigh fading occurs wherein parts of a transmitted signal stream may be subject to deep nulls in strength by the time the signals reach the receiver. Shadow fading may also occur when the transmitted signal is blocked by large structures such as buildings and bridges, and when a mobile receiver is beneath an overpass or in a tunnel. If program signals are broadcast in the form of packets of digital bits or symbols, then the packets or portions thereof are likewise subject to loss over a transmission path because of fading. In digital network (e.g., Internet) broadcasting, a signal bit stream corresponding to a program source is also divided into many packets, some of which may be lost or delayed during transmission through either wired or wireless portions of the network.
In digital broadcasting applications, feedback or re-transmission of lost symbols or packets are not desirable solutions, because of the nature of the broadcasting environment. One viable solution to combat fading or packet loss in a broadcast environment, is the use of forward error correction (FEC) schemes referred to as convolutional and block coding. These schemes protect the transmitted streams of source signals so that even if part of a signal stream is received with error, the error may be corrected or minimized using other transmitted information which is received under more favorable conditions.
Known block codes such as, e.g., Reed-Solomon (RS), Golay, BCH and the like, operate to correct errors occurring from relatively short noise or interference bursts over a signal transmission path. Depending on an amount of redundancy added to a source signal stream and a selected code block size, a given block code will allow up to a certain number of symbol errors to be corrected at the receiver.
Convolutional codes can work well to minimize reception errors due to relatively slow fading, provided appropriate interleaving (i.e., scrambling of a signal stream over a certain time interval) is also employed. In order for convolutional codes to work properly, consecutive symbols presented to a signal channel decoder at the receiver should be uncorrelated. For example, the consecutive symbols should be separated timewise from one another during transmission, such as occurs by interleaving. Accordingly, at the transmitter, an interleaver is typically used to scramble symbols from the source signal stream over a certain interleaver time delay or length, after the symbols have been subjected to block coding. The block-coded and interleaved symbols are then transmitted over, e.g., an assigned wireless broadcast frequency channel. At the receiver, received symbols are first de-interleaved, and then applied to a channel decoder.
Depending on Doppler effects on the signal transmission path, interleaving over a large number of signal symbols is often necessary for the entire transmission and reception process to work effectively. A large interleaver length will, however, result in large time delays when first tuning the receiver to a broadcast program, since decoding and reproduction of the original program source can start only after an entire interleaved packet has been buffered or stored at the receiver.
If block-coded symbols are interleaved over the duration of many blocks before transmission, symbols associated with a lost packet will be de-interleaved and found among many different coded blocks. Thus, the number of symbol errors that may occur in each coded block is reduced, and the likelihood that a selected block code will correct all symbol errors in a transmitted signal stream is correspondingly increased.
While a delay of as much as several seconds between signal reception and program reproduction at the receiver may go unnoticed by a user as long as the receiver remains tuned to one program, such delay can be intolerable when first tuning the receiver to select a program, or while re-tuning to select a different program. Then, delays of at most a fraction of a second may be tolerable. Thus, while large interleaver/de-interleaver lengths help to ensure that a program source signal stream is transmitted, received, decoded and reproduced with minimum error due to fading and noise, the interleaver lengths must be relatively short if program decoding delays at the receiver are to be minimized while the receiver is being tuned or switched among different programs.
A technique is provided that allows a receiver to reproduce a program source with minimal delay when the receiver is first tuned to the program source.
Broadly, accordingly to the invention, two signal streams associated with a program source are subjected to different delays resulting from error correction coding. A first source signal stream is subjected to a first correction delay to overcome expected signal errors for a given transmission channel. A second source signal stream is subjected to a second correction delay shorter than the first correction delay, or substantially no delay, to facilitate reproduction of the program source with minimal delay at a receiver when the receiver is first tuned to the transmission channel.
According to another aspect of the invention, a method of transmitting signals corresponding to a program source in a manner that facilitates reproduction of the program source with minimal delay at a receiver, includes interleaving a first signal stream associated with the program source using a first interleaver having a first length to overcome expected signal errors for a given transmission channel, thus producing a first interleaved signal stream, and interleaving a second signal stream associated with the program source using a second interleaver having a second length less than the first length of the first interleaver to facilitate reproduction of the program source with minimal delay at a receiver when the receiver is first tuned to the program source, thus producing a second interleaved signal stream. The interleaved signal streams are transmitted simultaneously with a determined timing relative to one another for reception by a receiver.
For a better understanding of the invention, reference is made to the following description take in conjunction with the accompanying drawing and the appended claims.